Water-soluble polymers containing pendent sulfonate and/or carboxylate groups have found wide utility as dispersants, scale and corrosion-preventing agents, detergent builders, sequestering agents, suspending agents, flame retardants, and the like. Copolymers containing both sulfonate and carboxylate groups are commonly prepared by copolymerization of unsaturated sulfonated salt monomers and unsaturated dicarboxylic acids or their salt or anhydride derivatives under free radical conditions. To date, however, it has not been possible to obtain substantially alternating copolymers containing approximately equimolar amounts of the comonomers and having reasonably high (i.e., greater than 1500) molecular weights. These difficulties are believed to be due to differences in monomer reactivity and the poor solubility of the comonomers in typical organic solvents.
For example, Jpn. Kokai No. 149,705/75 teaches that copolymers of high molecular weight but containing low levels (&lt;30 mol %) of olefinically unsaturated sulfonic acid recurring units may be produced by polymerizing maleic anhydride with vinyl sulfonic acid or methacrylsulfonic acid in the presence of benzoyl peroxide initiator.
Jpn. Kokai No. 213,714/84 discloses free radical copolymerization of maleic acid or maleic anhydride with an allylic compound such as sodium methallyl sulfonate in aqueous solution or suspension under acidic (pH=2.5-6.0) conditions. The maximum molecular weight obtained was only about 1250.
Jpn. Kokai No. 50064/79 teaches the attempted copolymerization of equimolar amounts of sodium allyl sulfonate and maleic anhydride in 95% ethanol using benzoyl peroxide initiator. The composition and molecular weight of the product obtained are not disclosed, however.
It is clear, therefore, that there is a need for methods by which substantially alternating 1:1 copolymers of olefinically unsaturated sulfonate salts and .alpha., .beta.-unsaturated dicarboxylic acid anhydrides having a substantial degree of polymerization may be prepared. Copolymers of this type and their derivatives would be expected to have unique and beneficial properties by virtue of the high proportion of pendent sulfonate salt groups present.